I'm wondering about this, really. Did Matt always plan this or did he really have an argument with Moffat during the 50th filming? Did he have a heart to heart with David Tennant because he was thinking of leaving and Tennant said 'It's better to leave when they'll miss you.'? Did the gap between this and series 8 push him toward the door?
I see no reason to give those rumours any serious consideration. Smith has done three complete seasons, which on average is how long most Doctors stick around. It's the length of terms held by Hartnell, Troughton, Davison, McCoy, Tennant and now Smith. Going out on a limb, I'd say Smith's contract expires at the end of the year and he's choosing not to renew it, feeling the time is right to move on with his career.